The present invention is concerned with electronically controlled braking systems for vehicles, commonly referred to as "Brake-by-Wire" (BBW) systems, of the type wherein control of the vehicle brakes is achieved by the use of electronic control signals generated at a brake pedal in response to a driver's braking demand and an electronic controller which is adapted to control the supply of fluid under pressure from a power source to the brakes in accordance with said electronic signals corresponding to the driver's demand.
In order to provide a responsive "feel" to the driver, conventional braking systems usually incorporate a brake pressure booster having a feature known as "jump-in". In a conventional vacuum boosted brake system, "jump-in" involves the use of engine generated power, in the form of vacuum, to apply the brakes to a predetermined pressure as soon as the driver moves the brake pedal far enough to operate the control valves controlling the supply of fluid to the brakes. This operation is illustrated in FIG. 1 of the attached drawings, where the pressure jumps to 10 bar at a pedal effort of 20 units. The pedal is relieved of the reaction loads during this stage of the operation, so that pedal effort, which would otherwise need to be increased beyond the valve-operating threshold in fixed proportion to the brake pressure, remains constant until the jump-in pressure is attained. This is typically between 3 bar-5 bar, but may be as high as lobar, and it seems to the driver as though the necessary pedal travel has occurred without any change in applied effort--hence the feeling of responsiveness. As soon as the required braking pressure is reached, the reaction force from the master cylinder returns the booster to a balanced state so that further pressure increase is controlled directly by the driver's additional pedal effort. Unwanted speed-dependent variations in, e.g. lining friction, can make high levels of jump-in unacceptable at low speeds, for example during parking, forcing compromise with regard to the chosen characteristics.
A further feature of conventional vacuum boosters is that they exhibit considerable hysteresis, so that during brake release the jump-in action is reversed at lower brake pressures than the original jump-in, and is not clearly felt by the driver. It is thereby possible to provide sensitive control of low-pressures for check-braking and especially for creep, e.g. on an inclined driveway or (in a car with automatic transmission) in heavy stop-go traffic.
Brake-by-Wire (BBW) systems suffer less hysteresis, and can achieve more rapid pressure changes than can a vacuum boosted system. These can enhance the general feeling of responsiveness, but they pose special difficulties during jump-in, which requires more subtle control of pressure rise at the beginning of braking than would a smooth characteristic.
Particular problems associated with the application of "jump-in" to Brake-by-Wire systems are:
1. The reverse jump-in action disturbs smooth control during normal check-braking since the operating pressure is much less affected by hysteresis. PA1 2. The enhanced control precision highlights the effects of brake lining-.mu. variation, causing further compromise. PA1 3. Overshoot control is difficult during a rapid increase to a low pressure because of the brake-caliper's non-linear volumetric displacement characteristics. PA1 means to raise the brake pressure to a first predetermined level, referred to as jump-in at a prescribed level of initial brake pedal travel; and PA1 means to vary the jump-in brake pressure level in accordance with vehicle speed. PA1 means by which the braking pressure is then raised to the jump-in level. PA1 means to raise the brake pressure to a first predetermined level, referred to as jump-in, at a prescribed level of initial brake pedal travel; PA1 means by which the brakes are pre-filled to a low pressure at an initial stage in the pedal travel, prior to jump-in, and maintained at that low level until jump-in is triggered; and PA1 means by which the braking pressure is then raised to the jump-in level. PA1 means by which the brakes are pre-filled to a low pressure at an initial stage in the pedal travel and maintained at that low level until actual brake demand exceeds the pre-fill pressure, and PA1 means causing the brake pressure thereafter to be controlled in accordance with said actual brake demand. PA1 means by which the brakes, having been pre-filled to said low pressure at an early stage in the pedal travel, are maintained at that low level until jump-in is triggered; and PA1 means by which the braking pressure is then raised to the jump-in level.
In accordance with a first aspect of the present invention there is provided an electronic braking system for an electronic braking system for a vehicle wherein control of the vehicle brakes is achieved by the use of electronic control signals generated at a brake pedal in response to a driver's braking demand and an electronic controller which is adapted to control the supply of fluid under pressure from a power source to the brakes in accordance with said electronic signals corresponding to the driver's demand, the system comprising:
Thus, in a system in accordance with the first aspect of this invention, the system is adapted to raise the brake pressure to a first predetermined level (jump-in) at a prescribed level of initial brake pedal travel, the jump-in brake pressure level being arranged to be variable with vehicle speed.
Advantageously, the system includes means by which the level of jump-in brake pressure is reduced at lower vehicle speeds and increased at higher vehicle speeds.
The system can also includes means to release the brake pressure at a second predetermined level, referred to as reverse jump-in which is lower than that at jump-in.
The system can further include means by which the brakes are pre-filled to a low pressure at an initial stage in the pedal travel, prior to jump-in, and maintained at that low level until jump-in is triggered; and
In accordance with a second aspect of the present invention, there is provided an electronic braking system for a vehicle wherein control of the vehicle brakes is achieved by the use of electronic control signals generated at a brake pedal in response to a driver's braking demand and an electronic controller which is adapted to control the supply of fluid under pressure from a power source to the brakes in accordance with said electronic signals corresponding to the driver's demand, the system comprising:
Thus, in a system in accordance with the second aspect of this invention, the system is adapted to raise the brake pressure to a first predetermined level (jump-in) at a prescribed level of brake pedal travel, the brakes being arranged to be pre-filled to a low pressure at an initial stage in the pedal travel and then maintained at that low level until jump-in is triggered, whereupon the braking pressure is raised to the jump-in level.
Advantageously, the system includes means to vary said jump-in brake pressure level in accordance with vehicle speed.
The system can also includes means by which the level of jump-in brake pressure is reduced at lower vehicle speeds and increased at higher vehicle speeds.
The system can also include mens to release the brake pressure, referred to as reverse jump-in, at a second predetermined level, which is lower than that at jump-in.
In accordance with a third aspect of the present invention, there is provided an electronic braking system for a vehicle wherein control of the vehicle brakes is achieved by the use of electronic control signals generated at a brake pedal in response to a driver's braking demand and an electronic controller which is adapted to control the supply of fluid under pressure from a power source to the brakes in accordance with said electronic signals corresponding to the driver's demand, the system comprising:
Thus, in a system in accordance with the third aspect of this invention, the brakes are arranged to be pre-filled to a low pressure at an initial stage in the pedal travel and maintained at that low level until actual brake demand exceeds the pre-fill pressure, whereupon the brake pressure is then controlled in accordance with actual brake demand.
Advantageously, the system can include means to raise the brake pressure to a first predetermined level, referred to as jump-in, at a prescribed level of initial brake pedal travel after the pre-fill step;
The system can also include means to vary said jump-in brake pressure level in accordance with vehicle speed.
The system can also include means by which the level of jump-in brake pressure is reduced at lower vehicle speeds and increased at higher vehicle speeds.
The system can further include means to release the brake pressure, referred to as reverse jump-in, at a second predetermined level, which is lower than that at jump-in.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings.